Various techniques are known for attaching a prosthesis to a patient's natural bone. Such techniques often involve the use of a screw having a head end adapted to be secured to the prosthesis and a point end adapted to be threaded or otherwise fixed into the patient's bone. To achieve reliable initial and long term fixation, it has been proposed, e.g., U.S. Pat. No. 5,360,448, to apply a porous coating to the screw shaft designed to accept bone ingrowth.
In an exemplary application, the screw head can be secured to a leg prosthesis and the screw point can be threaded into the patient's thigh bone, or femur. In such, and similar applications, the screw shaft projects percutaneously through the patient's outer skin layers and subcutaneous soft tissue. Over time, these skin layers grow over the natural bone but frequently produce sinus tracts around the screw shaft which are prone to marsupialization and infection, and which can lead to serious cases of osteomyelitis.
U.S. application US2004/0204686 A1 published 14 Oct. 2004, incorporated herein by reference, describes a use of porous material carried on a percutaneously projecting stud of an implantable device for promoting tissue ingrowth to avoid the formation of sinus tracts, form an infection and marsupialization resistant barrier, and enhance device anchoring.